1. Field of the Invention
The present invention relates to a base station apparatus and a communication terminal.
Priority is claimed on Japanese Patent Application No. 2011-078383, filed Mar. 31, 2011, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Currently, a standardization group for wireless interfaces such as 3GPP (third generation partnership project) aims to further improve the efficiency of frequency usage for third-generation systems such as W-CDMA (wideband code division multiple access), and thus advances standardization of successor systems (represented by LTE (long term evolution)) beyond the third generation.
In the wireless access system of LTE, OFDMA (orthogonal frequency division multiple access) is used for downlinks (i.e., data communication links from a base station to mobile stations). In OFDMA, a system band width is divided into multiple subcarrier units. In each subcarrier unit, a data channel is allocated to an individual terminal (i.e., mobile terminal called “UE”). Additionally, the configuration of the subcarrier units, and each terminal for the allocation, can be changed as time passes. Therefore, in an OFDMA system, allocation of physical channels can be flexibly performed by using two-dimensional wireless resources including frequency and temporal (time) components.
In LTE, information about temporally-variable data channel allocation for upstream and downstream links (i.e., uplinks and downlinks) is communicated to each terminal by using a downstream control channel called “PDCCH” (physical downlink control channel). In a downstream subframe (of LTE), whose data amount is computed by “14 OFDM symbols×the number of subcarriers included in the relevant band, a PDCCH and a PDSCH (physical downlink shared channel) are time-division multiplexed as shown in FIG. 9. A PDCCH region for PDCCH occupies 3 OFDM symbols or less measured from the head OFDM symbol, where the occupied size is not fixed. Therefore, in an LTE downlink, PCFICH (physical control format indicator channel) is used for informing each terminal of the number of OFDM symbols occupied as the PDCCH region, that is, information about the number of OFDM symbols required for PDCCH (see, for example, Non-Patent Documents 1 and 2).    Non-Patent Document 1: 3GPP TS 36.211 v9.1.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 9)”, pp. 54-57, March 2010    Non-Patent Document 2: 3GPP TS 36.213 v9.1.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 9)”, pp. 66-69, March 2010
Below, a PDSCH receiving procedure and a PUSCH (physical uplink shared channel) sending procedure, which are executed by a terminal, will be explained.
Step 1: PCFICH is received for each subframe, so as to obtain the information about the number of OFDM symbols required for PDCCH.
Step 2: PDCCH addressed to the present terminal is received (i.e., deciphered) using the information about the number of OFDM symbols for PDCCH, which was obtained in Step 1, so as to determine whether or not there is PDSCH/PUSCH allocation information assigned to the present terminal. If such allocation information assigned to the present terminal exists, it is contained (arranged) in a predetermined area (measured using the number of OFDM symbols for PDCCH and identification information (UEid) for identifying the relevant terminal) in the PDCCH region.Step 3: If it is determined in Step 2 that there is allocation information assigned to the present terminal, the allocation information is obtained, and reception of PDSCH and transmission of PUSCH are performed using the allocation information. If it is determined that there is no allocation information assigned to the present terminal, Step 3 is not executed.
However, the above-described conventional technique has the following problems 1 and 2.
Problem 1: This problem is degradation in reception quality of the control channel (PDCCH) when area extension is performed from a macro base station and a local base station (having lower transmission power than the macro base station). More specifically, there are problems for degradation in reception quality, as shown in an interference scenario A (see the left in FIG. 10) and an interference scenario B (see the right in FIG. 10).
Here, in case of PDSCH, relevant information is stored in a region selected within a PDSCH region so as to avoid interference. However, in case of PDCCH, relevant information is stored in a predetermined area (see the above Step 2), thereby easily receiving an interference effect from a base station (called an “interference-causing base station”).
Interference scenario A: A local base station (i.e., “CSG (closed subscriber group) station” which may be called a “CSG Home eNB” in this scenario A) permits an already-registered terminal (which has been registered with the present station and may be called “CSG UE”) to access the present station (such an access may be called “open”), and does not permit an unregistered terminal (which has not yet been registered with the present station and may be called “non-CSG UE”) to access the present station.
An anticipated problem in this scenario is that the reception quality for a control channel of the non-CSG UE, which is present near the CSG Home eNB, is degraded due to a strong interference from the CSG Home eNB.
That is, in this interference scenario A, the interference-causing base station is the CSG Home eNB, and a terminal (called an “interference-receiving terminal”) that receives the interference is the non-CSG UE which is present near the interference-causing base station and connected to a macro base station (called a “Macro eNB”).
Interference scenario B: A local base station (i.e., “non-CSG base station” which may be called a “Pico eNB” in this scenario B) in this scenario B does not apply a limitation to terminals which the local base station permits to access the station.
An anticipated problem in this scenario is that the reception quality for a control channel of a terminal (which may be called a “Pico UE” in this scenario), which accesses the Pico eNB in an area expanded by coverage expansion of the Pico eNB, is degraded due to a strong interference from the Macro eNB.
That is, in this interference scenario A, the interference-causing base station is the Macro eNB, and the interference-receiving terminal is the Pico UE which is present in the expanded area and connected to the Pico eNB.
Problem 2: This problem is large battery consumption on the terminal side due to an operation of “PCFICH reception→PDCCH reception→PDSCH reception→PUSCH transmission (if receiving allocation information assigned to the present terminal)” executed for each sub-frame a shown in the above Step 1 to Step 3. This problem does not limitedly relate to a case of area extension from a macro base station and a local base station.